The present invention relates to an extrusion-molded honeycomb material used, for example, as a catalyst carrier for automobile exhaust gas purification, as well as to a process for producing the honeycomb material.
In recent years, the regulation of exhaust gases has become stricter for environmental protection. In this connection, a catalyst for exhaust gas purification having a higher purification ability is required. Meanwhile, an engine of lower fuel consumption and higher output is required strongly. To respond to such a situation, the catalyst for exhaust gas purification is required to show lower pressure loss in addition to the higher purification ability.
To satisfy these requirements, it is vigorously desired to allow the honeycomb structure used in the catalyst for exhaust gas purification to have a thinner partition wall so that the honeycomb structure can enable easier gas flow and lower pressure loss and the catalyst can have a lighter weight and lower heat capacity and can have a higher purification ability during engine warm-up. The partition wall thickness of honeycomb structures has heretofore been 150 xcexcm (6 mil) mainly, but it is shifting to 50 xcexcm (2 mil) mainly. Incidentally, xe2x80x9choneycomb structurexe2x80x9d refers to a structure in which a large number of cells are separated by partition walls.
A honeycomb structure is ordinarily produced by mixing a raw material powder (e.g. a ceramic powder or a metal powder) with a binder or the like, subjecting the resulting mixture to extrusion molding through a die having lattice-like slits, and drying and firing the resulting extrudate. As the above binder, there has been used a water-soluble thermosetting methyl cellulose type binder.
As the partition wall of honeycomb structure becomes thinner, the width of the slits of the die needs to be smaller. Therefore, in molding a honeycomb structure having a thin partition wall, it is required to use a binder of high fluidity which can quickly flow into a die. Further, as the partition walls of honeycomb structures become thinner, the fresh extrudate from the die has a lower strength and tends to deform due to its own weight. Therefore, it is necessary to use a binder of high shape retainability which can solidify quickly after leaving the die.
Hence, it has been conducted to mold a honeycomb structure by using a molding material of high hardness and higher shape retainability, or a molding material of low hardness and high fluidity. Such molding materials, however, have had the following problems. A molding material of high hardness is inferior in fluidity; therefore, (1) it does not flow into a die easily, resulting in low productivity and (2) it requires a high molding pressure and repeated molding brings about deformation or wear of the die.
A molding material of low hardness must be hardened by thermal gelation of the binder contained in the molding material, by way of dielectric drying, to allow the extrudate from the die to have a desired strength. In this case, the transfer of the extrudate to a dielectric dryer is conducted with a gas flow applied to the extrudate from below the extrudate to prevent the deformation of the extrudate caused by its own weight. As a result, the extrudate comes to have cracks caused by drying at the portion to which the gas flow is applied.
In view of the above situation, the present invention aims at providing a molded honeycomb material having a thin partition wall and a process for mass-producing such a honeycomb structure without impairing the honeycomb quality.
According to the present invention, molded honeycomb material is obtained by subjecting a mixture of a raw material powder and a binder to extrusion molding. No open pores are present in the honeycomb material, and the binder comprises a thermoplastic material which is molten at the molding temperature.
In the molded honeycomb structure of the present invention, the binder is preferably water-insoluble and is preferably a wax, a thermoplastic resin or a mixture thereof.
When the binder is a mixture of a wax and a thermoplastic resin, the mixing ratio of the thermoplastic resin in the binder is preferably 35 to 80% by weight, more preferably 40 to 70% by weight, further preferably 45 to 60% by weight. The raw material powder can be a ceramic powder (e.g. cordierite) or a metal powder.
The molded honeycomb material of the present invention can be used as a carrier for the catalyst for removing harmful substances and dust from an automobile exhaust gas.
According to the present invention, a process for producing the above-mentioned molded honeycomb material is also provided, comprising the steps of heating a mixture of a raw material powder and a binder to a molding temperature to melt the binder, subjecting the mixture to extrusion molding, and cooling and solidifying the extrudate.
The molded honeycomb material of the present invention can be produced by subjecting a mixture of a raw material and a binder to extrusion molding. As the binder, a thermoplastic material is used which is molten at the molding temperature.
The thermoplastic material can be melted by heat and can have different viscosities at different temperatures. Therefore, it can have the desired fluidity by appropriately selecting the temperature which enables efficient mass production of a molded honeycomb material.
The molten thermoplastic material solidifies when cooled. Therefore, the binder can be easily solidified by rapidly cooling the extrudate with cold water, cold air or the like, before the extrudate deforms due to its own weight, whereby the extrudate can retain its shape.
In the present invention, a water-insoluble binder is used, which makes drying of the molded material unnecessary. In extrusion molding of a mixture of a raw material powder and a water-soluble binder (a mixture of a raw material and a binder is hereinafter called xe2x80x9craw material mixturexe2x80x9d), foam in the raw material mixture must be removed by vacuum defoaming, and local drying taking place during vacuum defoaming hardens the dried portion and causes plugging of the die. In contrast, in extrusion molding of a mixture of a raw material powder and a water-insoluble binder, no local drying takes place, no plugging of the die takes place, and productivity is high. Incidentally, no vacuum defoaming is necessary in extrusion molding using a water-insoluble binder.
In the present invention, the specific water-insoluble binder is preferred to be a wax or a thermoplastic resin. As the wax, paraffin wax, microcrystalline wax, etc are preferred. As the thermoplastic resin, ordinary thermoplastic resins such as EVA, polyethylene, polystyrene, liquid crystal polymer, engineering plastics and the like are preferred. In the present invention, these binders can be used singly or in combinations of two or more kinds. An auxiliary agent such as a coupling agent, lubricant, dispersing agent or the like may be added to the binder.
In the present invention, when a mixture of a wax and a thermoplastic resin is used as the binder, the mixing ratio of the thermoplastic resin in the binder is preferably 35 to 80% by weight, more preferably 40 to 70% by weight, further preferably 45 to 60% by weight.
The reason why the above mixing ratio of the thermoplastic resin is preferred is that the amount of the thermoplastic resin in the binder influences the shape retainability and molding pressure during molding, as well as the amount of expansion, amount of cracks and adhesion to the setter during dewaxing and firing.
As the amount of the thermoplastic resin in the binder becomes larger, the shape retainability during molding is better, the molding pressure required is higher, and the amount of expansion and number of defects during dewaxing and firing are lower.
For the above reasons, the upper limit of the mixing ratio of the thermoplastic resin in the binder is set preferably at 80% by weight, more preferably at 70% by weight, further preferably at 60% by weight. Thereby, the shape can be retained and an increase in molding pressure can be prevented. As a result, a molded honeycomb material having a small partition wall thickness and large cell density can be satisfactorily obtained with no deformation of the extruder die.
When the mixing ratio of the thermoplastic resin in the binder is too large, the extrudate has a high temperature (a large heat stress), reducing the handleability.
In the present invention, the mixing ratio of the thermoplastic resin in the binder is set preferably at 35% by weight, more preferably at 40% by weight, further preferably at 45% by weight. Thereby, the amount of expansion, the amount of cracks and adhesion to setter during dewaxing and firing can be reduced.
In the present invention, the mixture of the raw material powder and the binder is extrusion-molded. In the mixture, the amount of the binder differs depending upon the kind of the binder used and the binder is added in such an amount that desired fluidity can be obtained.
In the present invention, a ceramic powder or a metal powder is preferably used as the raw material powder. As the ceramic powder, a powder of an oxide (e.g. cordierite, alumina or mullite) or a nitride ceramic (e.g. silicon nitride, silicon carbide or aluminum nitride) can be used. As the metal powder, a powder of Fe, Cr, Ni, Al or the like can be used.
In the present invention, a molded honeycomb material is produced by extrusion molding. The kneading apparatus used therein can be any apparatus as long as it allows heating and pressurization, and there can be used an ordinary kneader, a pressure kneader, a twin-screw continuous kneader and extruder or the like.
The molding apparatus used in the present invention can be any apparatus as long as it allows heating and pressurization and has an extrusion function. An extruder of plunger type, a pug mill, an injection molding machine, a single-screw continuous extruder, a twin-screw continuous kneader or the like can be used.
In the present invention, kneading and molding may be conducted simultaneously using, for example, a twin-screw continuous kneader and extruder which can conduct kneading and molding simultaneously.
In continuous molding, it is necessary to atomize the binder. The atomization can be conducted, for example, by spray-drying or freeze-grinding. There is no restriction as to the heating means of the molding apparatus, and the heating means may be a heater or circulation of a heating medium such as oil or the like.
The binder is appropriately selected depending upon the cell structure of the desired honeycomb. A molded material of smaller partition wall thickness and smaller cell density must have a larger strength for shape retention. In this case, therefore, the binder used therein needs to contain a higher content of a thermoplastic resin relative to a wax. For example, in producing a molded material having a partition wall thickness of 12 mil and a cell density of 300 cells/in2, the binder can consist of a wax alone and molding is possible. In producing a molded material having a partition wall thickness of 4 to 1 mil and a cell density of 500 to 1,200 cells/in2, however, a mixed binder of a wax and a thermoplastic resin is used, wherein the content of the thermoplastic resin is preferably 35 to 80% by weight, more preferably 40 to 70% by weight, further preferably 45 to 60% by weight. Needless to say, a satisfactory honeycomb material can be obtained even with a thermoplastic resin alone.
The molding temperature of the raw material mixture is determined by the kind of binder used. The molding temperature is about 60 to 100xc2x0 C. when the binder is, for example, a wax alone or a mixture of a wax and an EVA. When a high-melting point thermoplastic resin is used, the molding temperature is about 280xc2x0 C. in the case of a polyethylene, and about 350xc2x0 C. in the case of a liquid crystal polymer. Use of a low melting point binder is favorable in view of handling and thermal stress considerations of the extrudate.
The kneading and molding temperature need be controlled so that the binder is not deteriorated.
The viscosity of the raw material mixture is determined depending upon the kind and amount of binder used and the molding conditions (temperature and pressure) selected, and is in a range wherein a honeycomb material can be molded. The amount of binder can be appropriately set depending upon the kind of the raw material powder.
In the present invention, the extrudate is cooled and solidified to prevent deformation of the extrudate. There is no restriction as to the method of cooling, and air cooling, water cooling (spraying) or the like can be used. Alternatively, the extrudate may be dropped into water for rapid cooling. Forced cooling is unnecessary depending upon the molding temperature used. The extrudate is pushed out from an extrusion molding machine ordinarily in a horizontal direction, but may be pushed out downward using a vertical molding machine.
The cooling temperature may be a temperature at which the binder solidifies. The difference between the molding temperature and the cooling temperature is preferred to be small in view of the small stress during cooling. The cooling rate is preferred to be small.
In the present invention, there is no particular restriction as to the sectional shape of the cell of the molded honeycomb material produced. The cell sectional shape may be a polygon (e.g. triangular, rectangular or hexagonal), a circle or the like. The cell density may be 300 to 2,000 cells/in2.
The firing of the molded material is conducted, at low temperature ranges, under conditions where no cell cutting takes place, in view of the vaporization curve of the binder and, at high temperature ranges, under conditions where the intended porosity and thermal expansion coefficient, etc. can be achieved.
The dewaxing and firing of the molded honeycomb material can be conducted in an atmosphere (e.g. air, inert atmosphere or vacuum) which is appropriately selected depending upon the kind of the raw material powder used.
For example, when the raw material powder is a cordierite powder (an oxide), dewaxing is conducted in air and then firing is conducted in air. Ordinarily, dewaxing and firing are conducted simultaneously in a periodic kiln or a continuous kiln such as a tunnel kiln or the like.
In using the honeycomb structure produced, as a catalyst for automobile exhaust gas purification, a xcex3-alumna layer is formed on the cell partition wall. In the pores of the xcex3-alumna layer is supported a catalyst component (i.e., a noble metal such as platinum, rhodium, vanadium or the like) and the catalyst component is baked at a temperature of about 600xc2x0 C.